Piston ring coating

ABSTRACT

A system for preventing microwelding of a piston ring installed in a circumferential groove of a piston, wherein the ring has at least one piston groove engaging surface which is coated with thermal set resin, polytetrafluoroethylene and molydisulfide. This composition provides critical lubrication during the piston and piston ring break-in period and prevents localized high pressure, and high temperature areas which promote microwelding.

FIELD OF THE INVENTION

The present invention relates to lubricant compositions and moreparticularly to lubricant compositions for use in coating piston ringsinstalled in internal combustion engines.

BACKGROUND OF THE INVENTION

Great strides have been made in extending the useful life of theinternal combustion engine. Many of these improvements have been madepossible by utilizing materials which reduce the friction between movingcomponents used within the internal combustion engine. For example,coating the cylinder wall engaging surface of a piston ring withpolytetrafluoroethylene (PTFE) to reduce the sliding friction betweenthe piston ring and the cylinder wall is a known technique for reducingengine friction. It is also known to deposit PTFE between the rodbearings and the respective crank journal faces of an internalcombustion engine to minimize friction.

In spite of these advancements, there still remains several frictionintense areas of conventional engine designs which have not beenadequately solved using even the most advanced friction reducingcoatings. One such problem area relates to the interface between pistonsand piston rings of internal combustion engines.

In the vast majority of internal combustion engines which usereciprocating pistons, the pistons are surrounded by piston rings tocreate a relatively efficient gas seal between the piston and thecylinder wall. Thus, when a charge within the engine cylinder isignited, creating high combustion chamber pressures, the expandinggasses which are formed during the burning process are confined to thecombustion chamber. The confined gases exert a downward force on thepiston and are not permitted to escape between the piston and thecylinder wall. Although the piston ring is typically captured within agroove which is cut along an outside circumferential surface of thepiston, the ring is sized relative to the groove so that is free torotate within the groove. It is important that the piston ring bemovable with respect to the groove because its relative movement resultsin a more uniform heat transfer between the piston and the cylinderwalls.

There is a critical time period for new engines known as the break-inperiod during which the moving surfaces of joined components adjust tomatingly conform to one another. During this break-in period thepiston/piston ring interface is particularly susceptible to a conditionknown as microwelding, wherein there is a propensity for the piston ringto stick to the side walls of the piston groove. Microwelding is aphenomenon which causes the areas of contact between the piston and thepiston ring to literally weld to one another as a result of the pressureand temperature experienced by the contact points. If during break-in,the piston ring can be kept free to move relative to the walls of thepiston groove, the piston groove/piston ring will mate uniformly andmicrowelding will not occur.

SUMMARY OF THE INVENTION

The present invention is directed to a system for preventingmicrowelding of a ring to a piston and includes a ring which is coatedwith a composition of thermal set resin, polytetrafluoroethylene, andmolydisulfide.

The present invention is also directed to a system for preventingmicrowelding of a ring to a piston and includes a piston which isadapted to reciprocate within a combustion chamber of an engine. Thepiston has an outer surface with a circumferential groove disposedtherein. A ring is disposed within the circumferential groove, the ringincluding a cylinder wall engaging surface and a piston groove engagingsurface. The piston groove engaging surface is coated with a compositionof thermal set resin, polytetrafluoroethylene, and molydisulfide.Preferably, the ring includes a radially extending upper surface and aradially extending lower surface, wherein the piston groove engagingsurface includes the lower surface of the ring.

The present invention is also directed to a method for preventingmicrowelding of a piston ring to a piston. A composition comprisingthermal set resin, polytetrafluoroethylene, and molydisulfide isdeposited on a surface of a piston ring. The composition is preferablycured by exposing the piston ring to an elevated temperature for apredetermined period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and inventive aspects of the present invention will becomemore apparent upon reading the following detailed description, claims,and drawings of which the following is a brief description:

FIG. 1 is the piston ring of the present invention installed in acircumferential groove of a piston.

FIG. 2 is a first embodiment of the piston ring of the presentinvention.

FIG. 3 is a partial cross-sectional view taken along lines 3--3 of FIG.1.

FIG. 4 is a second embodiment of the preferred piston ring of thepresent invention.

FIG. 5 is a graphical representation of comparative test results.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred system 10 of the present invention, as shown in FIG. 1, apiston 12 is fitted with at least one circumferential groove 13 (seeFIG. 3). Groove 13 is defined by upper and lower radially extendingwalls 18, 20 respectively and a vertical wall 22. A piston ring 14 istypically installed within groove 13. It is not uncommon for piston 12to have two or more rings 14, 14', 14" to ensure efficient sealing ofcombustion chamber gasses and to also ensure the minimal flow oflubricating oil into the combustion chamber from the engine crank case.

As shown in FIG. 3, a first embodiment of piston ring 14 includes upperand lower radially extending surfaces 24, 26, a radially inner verticalsurface 28, and a radially outer cylinder wall engaging surface 30.Lower surface 26 is coated with a composition 32 of thermal set resin,polytetrafluoroethylene and molydisulfide. In a second embodiment, shownin FIG. 4, the entire outside surface offing 14 is coated with acomposition 32' of thermal set resin, polytetrafluoroethylene andmolydisulfide. The composition is currently marketed under the tradename of Xylan 1620™, Whitford Corporation, Box 507, Westchester, Pa.19381-0507 (phone number 215-436-0600).

Xylan 1620™ is normally applied to parts to reduce wear. In the case ofthe present invention, however, it is used to permit ring 14 to moverelative to walls 18, 20 and 22 of piston groove 13 and thereby toprevent microwelding. In some extremely demanding applications, it maybe necessary to coat piston ring surfaces 24, 26, 28, and 30 with Xylan1620™. However, current testing indicates that in most applications itis only necessary to coat lower engagement surface 26 of piston ring 14to prevent microwelding. After the break-in period has expired, theXylan 1620™ is usually worn away and plays no further role.

Xylan 1620™ prevents microwelding from occurring by allowing the pistongroove surfaces and the piston ring to move relative to one another.Thus, uniform mating is promoted throughout the critical engine break-inperiod. Xylan 1620™ reduces or eliminates direct contact between ring 14and piston groove walls 18, 20, and 22. Piston ring 14 is typicallyformed from ductile cast iron or steel. Piston 12 is typically formedfrom aluminum. By separating the dissimilarly composed piston 12 andring 14 during the critical break-in period, the heat transfer betweenthem is made uniform at the interface between ring 14 and walls 18, 20and 22 of the piston groove. Thus, the two surfaces conform to oneanother without the high localized pressures and temperatures whichmight otherwise be experienced if not for the presence of the Xylan1620™.

It is preferred that the Xylan 1620™ be applied to the ring surface orsurfaces rather than walls 18, 20, 22 of piston groove 13. It is mucheasier to control the thickness and the placement of the Xylan 1620™coating when the Xylan 1620™ is deposited on the ring as opposed to thewalls of the piston groove. Additionally, when applied piston 12, Xylan1620™ may act as a thermal barrier, resulting in too much heatresistance in the piston and possible piston failure. Further, if thesofter aluminum of piston groove 13 were coated, Xylan 1620™ mayundesireably separate from walls 18, 20, and 22 during the criticalbreak-in period.

Preferably, the Xylan 1620™ is applied to the piston ring by way ofdipping, spraying, or rolling. Thereafter, the ring is cured in an ovenin accordance with the recommendations of the manufacturer of the Xylan.In tests discussed further below, the inventors applied the Xylan 1620™by means of dipping and then cured the rings for approximately tenminutes at an approximate temperature of 232° C.

The following example will serve by way of illustration and not by wayof limitation, the system of the present invention and the resultsobtained thereby in comparison to other materials.

As summarized in FIG. 5, the inventors conducted a series ofexperiments. For each experiment the inventors used six experimentalpiston rings installed in the upper groove of six pistons in a sixcylinder internal combustion engine. A conventional piston ring wasinstalled in the lower groove of each piston. The pistons, experimentalpiston rings, and lower conventional piston rings were replaced aftereach experiment. Each experimental piston ring had a diameter of 3.7inches (94 mm), an edge width of 5/64 inches (2 mm), and a radial walldimension of 0.136 inches (3.5 mm). The experimental rings were allformed from a ductile cast iron.

In a control experiment no coating was used. All six experimental pistonrings exhibited microwelding within the baseline time of thirty minutes.

In a second set of experiments the inventors used various hard coatings.A first experiment involved a surface coating of chrome plating appliedusing an electrolytic bath. A second experiment involved a surfacecoating of titanium nitride using an ion plating method known in theart. A third experiment involved a surface coating of chromium nitrideusing an ion plating method known in the art. A fourth experimentinvolved a surface coating of gas nitriding wherein the gas nitride wasapplied using a gas furnace. In each of the four experiments all sixexperimental piston rings exhibited microwelding within the baselinetime of thirty minutes.

In a third set of experiments the inventors used various solidlubricants. A first and second experiment involved molydisulfide basedcoatings sold by Dow Coming under the trade names Molykote D10™ and106™, respectively. For each experiment the coatings were sprayed ontothe six experimental piston rings and cured for approximately an hour atapproximately 150° C. A third experiment involved the spraying oftungsten disulfide onto the experimental piston rings. In each of thethree experiments an average of four rings exhibited microwelding withinthe baseline time of thirty minutes.

In a fourth set of experiments the inventors used various liquidlubricants. A first experiment involved dipping the experimental pistonrings into engine oil. A second and a third experiment involved dippingthe experimental piston rings into two different products sold under thetrade name Biotron™. The first was an engine formulation and the secondwas a penetrating lubrication. In each of the three experiments all sixrings exhibited microwelding within the baseline time of thirty minutes.

In a fifth set of experiments the inventors coated the experimentalpiston rings with a composition of thermal set resin,polytetrafluoroethylene and molydisulfide. The composition is currentlymarketed under the trade name of Xylan 1620™. The Xylan 1620™ wasapplied by means of dipping the experimental piston rings into the Xylan1620™ and then curing the rings for approximately ten minutes at anapproximate temperature of 232° C. As shown in FIG. 5 none of the ringsexhibited microwelding after the baseline time of thirty minutes. Onlyone ring displayed microwelding after 13 hours. No additionalexperimental piston rings exhibited microwelding even after 21 hours.

In a sixth set of experiments the inventors sprayed the experimentalpiston rings with polytetrafluoroethylene and then cured the rings forapproximately ten minutes at an approximate temperature of 232° C. Threeof the six rings exhibited microwelding within the baseline time ofthirty minutes.

Preferred embodiments of the present invention have been disclosed. Aperson of ordinary skill in the art would realize however, that certainmodifications would come with the teachings of this invention.Therefore, the following claims should be studied to determine the truescope and content of the invention.

What is claimed is:
 1. A system for preventing microwelding of a pistonring to a piston, comprising:a piston ring having a surface coated witha composition of thermoset resin, polytetrafluoroethylene, andmolydisulfide.
 2. The system of claim 1, wherein said piston ringincludes a cylinder wall engaging surface and at least one piston grooveengagement surface wherein only said piston groove engagement surface iscoated with said composition.
 3. The system of claim 2, wherein saidpiston ring includes an upper radially extending surface and a lowerradially extending surface, wherein said piston groove engaging surfaceof said piston ring comprises said lower surface.
 4. The system of claim1, wherein said ring is comprised of cast iron.
 5. The system of claim1, wherein said ring is comprised of steel.
 6. A system for preventingmicrowelding of a piston ring to a piston, comprising:a piston adaptedto reciprocate within a combustion chamber of an engine, wherein saidpiston includes walls extending radially inwardly from an outer radialsurface of said piston, said walls defining a circumferential groove;and a piston ring disposed within said circumferential groove, said ringincluding a cylinder wall engaging surface and at least one pistongroove engaging surface, wherein at least one of said piston grooveengaging surface of said ring and said circumferential piston groove iscoated with a composition of thermoset resin, polytetrafluoroethylene,and molydisulfide.
 7. The system of claim 6, wherein said piston iscomprised of aluminum.
 8. The system of claim 7, wherein said ring iscomprised of one of cast iron and steel.
 9. The system of claim 6,wherein said ring includes an upper radially extending surface and alower radially extending surface, wherein said piston groove engagingsurface comprises said lower radial surface.
 10. A system for preventingmicrowelding of a ring to a piston, comprising the steps of:(A)depositing on a surface of a piston ring a composition comprisingthermoset resin, polytetrafluoroethylene, and molydisulfide, (B) curingsaid composition by exposing said piston ring to an elevated temperaturefor a predetermined period of time.
 11. The system of claim 7, whereinstep A further includes only depositing said composition on a bottomportion of said piston ring.